1. Field of the Invention
The present invention relates to the manufacture of semiconductor devices and, more particularly, to thickness detection systems used during chemical-mechanical polishing of an integrated circuit substrate.
2. Description of the Prior Art
In certain technologies, such as integrated circuit fabrication, optical device manufacture, and the like, it is generally crucial to the various fabrication processes for the workpiece from which the integrated circuit, optical or other device is to be formed have a substantially planar front surface and, for certain applications, have both a planar front surface and back surface.
One process for providing a planar surface is commonly referred to as mechanical polishing. In mechanical polishing, the surface of the substrate is scoured with a conformable polishing pad. When a chemical slurry is used in conjunction with the polishing pad, the combination of the chemical slurry and the polishing pad generally provides a higher material removal rate than is possible with mere mechanical polishing. This combined chemical and mechanical, commonly referred to as “CMP”, is considered an improvement over mere mechanical polishing processes for polishing or planarizing substrates. The CMP technique is common for the manufacture of semiconductor wafers used for the fabrication of integrated circuit die.
CMP typically involves mounting a semiconductor wafer (substrate) face down on a holder and rotating the wafer against a polishing pad mounted on a platen or a belt. The platen is, in turn, rotated or in an orbital state. A slurry containing a chemical that chemically interacts with the facing wafer layer and an abrasive that physically removes that layer is caused to flow between the wafer and the polishing pad or on the pad near the wafer. This technique is commonly applied to planarize metalization layers in the semiconductor wafer, but is applicable to oxide layers or films on the wafer such as silicon dioxide.
Device geometries for semiconductor integrated circuits are becoming increasingly smaller. As a result, the smaller devices tend to cause a variety of problems. One such problem is the contact via alignment between an upper conductive layer and a lower conductive layer. The contact alignment problem between these conductive layers is often caused by a non-uniform inter-dielectric layer therebetween. The non-uniform inter-dielectric layer is often characterized by peaks and valleys defined in the layer surface. This in turn causes other fabrication problems with each succeeding layer. The CMP process is used to reduce these layer non-uniformities.
In the CMP process, it is critical to utilize end-point detection to determine when and how much of the desired film is removed and to terminate the polishing process, thus minimizing the damage to the device performance that overpolishing will cause. The problem with overpolishing is that the film will be thinner than planned and modeled so the device may not act as planned. Semiconductor wafers and flat panel displays are particularly sensitive to overpolishing and damage to the underlying layer. CMP is performed in the processing of semiconductor wafers and/or chips on commercially available polishers, such as the SF776 & 676, AMT Mirra, and LAM Teres polishers. The standard CMP tools have a circular polishing table and a rotating carrier for holding the substrate.
End-point detection has been determined by motor current measuring mechanisms. As well, end-point detection has been accomplished by measuring the film thickness optically by interferometer, ellipsometer, or light beam displacement. If the film is opaque, the difference in reflectivity between the film being etched or polished and the film beneath it can be used to detect the end point of the process (or simply “endpoint”). The presence of a reaction product or the absence of a reacting species can be monitored and used to determine when the film etching is complete.
Another method utilized for end-point detection is detecting a change in process pressure. Monitoring process pressure, however, is usually not a very sensitive method of detecting end-points. Typically, current systems actively control process pressure, thus masking any pressure changes at end-point.
During the CMP process of silicon dioxide films on semiconductors, it is desirable to know the thickness of the film as it is being polished. By knowing the thickness of the film, the control and process capability of the tool set is enhanced. There are many systems to perform end-point measuring on flat wafers, or wafers with relatively simple film stacks, such as shallow trench isolation, or potentially the first oxide dielectric over a gate metal. However, there are no end-point systems than can reliably end-point polish in production after two, three, four, five, or more layers of metal (i.e. complex film stacks) have been processed.
Currently, all of the oxide end-point systems currently used in production, are optically based. Of these optically based systems, some use broad band visible light, such as IPEC, Speedfam, and Lam, while other systems use a single wavelength laser system such as AMAT. These systems use some form of either through the pad or off the pad optical sensing that have several features in common which prohibit them from measuring thickness on complex film stacks.
A problem with these systems is trying to determine the appropriate signal by reading an area of the semiconductor from a few square millimeters to a few square centimeters. The signal coming from this area will be quite complex, since it can have up to and over a dozen different oxide thicknesses in the area under evaluation. Currently, there are no CMP systems that are capable of accurately performing end-point on semiconductors with a plurality of different oxide thicknesses.
Another problem with the above optical systems is the rate at which the systems operate. Typically, the semiconductor wafer surface is moving at a radial velocity that translates to up to a few hundred linear feet per minute. Such conventional image capturing (optical) systems will not capture a sharp image with such great velocities.
What is needed is a method of determining thickness of films on a substrate during CMP.
What is also needed is a method of determining when an end-point is reached for substrates having a plurality of layers.
What is further needed is process of end-point detection for semiconductors having a plurality of oxide layers each of which may have a different thickness.
What is even further needed is a process of end-point detection during CMP of silicon dioxide films on semiconductors.